1. Technical Field
The present invention relates to a tractor unit that conveys continuous paper by sequentially engaging engagement pins in feed holes formed along the length of the continuous paper and moving the engagement pins.
2. Related Art
Printers that convey continuous paper through a paper feed path past the printing position of an inkjet head or dot impact printhead, and print on the continuous paper at the printing position, are known from the literature. Such printers commonly have a tractor unit and paper feed roller as a paper conveyance mechanism for conveying the continuous paper. The tractor unit conveys continuous paper by sequentially engaging engagement pins in feed holes formed along the length of the continuous paper on both sides of the paper width and moving the engagement pins. The paper feed roller is disposed downstream in the conveyance direction from the tractor unit and driven on an axis of rotation perpendicular to the conveyance direction of the continuous paper to convey the continuous paper. Printers in which the platen roller that determines the printing position functions as the paper feed roller are also known.
A tractor unit that can be used in such printers is described in Japanese Unexamined Patent Appl. Pub. JP-A-2002-296855. The tractor unit taught in JP-A-2002-296855 has a pair of tractors that hold the continuous paper from both sides of the paper width with the engagement pins engaged in the feed holes of the continuous paper, two shafts that extend across the paper width and pass through the pair of tractors, and a clamping member that holds one of the two tractors at a specific position on the paper width.
The tractor unit described in JP-A-2002-296855 is used in a laser printer that transfers a toner image formed on a photosensitive drum to the continuous paper, and fuses the toner image on the continuous paper by applying heat and pressure. A problem with this laser printer is that the applied heat may shrink the continuous paper, causing the engagement pins to disengage the feed holes and resulting in the recording paper jamming. The tractor unit described in JP-A-2002-296855 therefore provides a gap between the clamping member and the tractor that is held by the clamping member, allowing the tractor to move on the clamping member an amount equal to the size of the gap widthwise to the paper. As a result, when the continuous paper shrinks, the one tractor slides widthwise to the paper according to the shrinkage of the paper on the clamp, the engagement pins follow the movement of the feed holes, and the feed holes are prevented from separating from the engagement pins.
Separation of the feed holes in the continuous paper from the engagement pins is not limited to when the continuous paper shrinks due to the printing method, and can also happen during normal conveyance of the continuous paper in printers with a tractor unit. The continuous paper can also become deformed and jam in the tractor unit. For example, if the conveyance direction of the continuous paper by the paper feed roller and the conveyance direction of the continuous paper by the tractor unit do not match due to the dimensional precision of the parts used in the paper feed roller and tractor unit, or the installation precision of the paper feed roller and tractor unit in the printer, for example, the conveyed continuous paper will become skewed and the force causing the continuous paper to move in a direction skewed to the conveyance direction causes the feed holes to disengage the engagement pins. If the conveyed continuous paper becomes skewed, the continuous paper will be deformed by the force causing the continuous paper to move in a direction skewed to the conveyance direction, and may jam.
The tractor unit described in JP-A-2002-296855 could conceivably be used to prevent the feed holes from separating from the engagement pins due to the installation precision of the paper feed roller and tractor unit, for example, but the construction of the tractor unit taught in JP-A-2002-296855 is complicated by the the need to use a clamping member that supports the tractor movably transversely to the paper.